The reduction of leakage losses in turbines has been the subject of intensive development work for several decades. During operation of a gas turbine, relative movement between the rotor and the housing is unavoidable. The resultant wear of the housing or wear of the blades has the effect that a sealing action is no longer provided. As a solution to this problem, a combination of thick coatings which can be ground away on the heat shield and abrasive protective coatings on the blade tips is provided.
Methods for applying additional coatings to blade tips or for increasing the resistance to wear by suitable modification of the blade tip have been known even since the 1970s. Various methods have likewise been proposed for simultaneously making such protective coatings resistant to frictional contacts and oxidation caused by the hot gas by a combination of abrasive particles (carbides, nitrides, etc.) with oxidation-resistant materials. Many of the proposed methods are expensive and complex to implement, however, making commercial use more difficult. This applies above all also to the application of these methods for single-crystal turbine blades. Here, additional demands are made on the production process because of the special single-crystal microstructure of the basic material. In particular, the production of the abrasive blade tip should have the smallest possible influence on the basic body of the turbine blade. This includes the avoidance of defects at the interface with the basic body of the blade.
One of the popular strategies therefore lies in dispensing entirely with the protection of the blade tip against wear and providing the heat shield with special porous, ceramic rub-in coatings. Owing to their high porosity, these can also be rubbed in to a certain extent by unprotected blade tips. However, considerable technical risks are associated with this method, since the porous, ceramic rub-in coatings do not ensure the same resistance to erosion as dense coatings. A further risk lies in operational changes to the porous, ceramic rub-in coatings (densification by sintering), which can have a negative effect on the tribological properties. For this reason, a combination with wear-resistant (abrasive) blade tips is expedient when using ceramic protective coatings on heat shields.
Turbine blades with abrasive coatings have been known since the 1960s. In recent decades, a plurality of methods for producing abrasive blade tips have been developed as shown, for example, in U.S. Pat. No. 6,194,086 B1. Although the use of laser metal forming (LMF) to build up abrasive blade tips has been known since the start of the 1990s (see for example DE 10 2004 059 904 A1), this method is still used rarely on an industrial scale.
EP 1295969 A1 and EP 1295970 A1 disclose MCrAlY materials which are used as coatings which protect against oxidation, have a relatively large surface area and have the best possible adaptation to a single-crystal basic material.
The (epitaxial) E-LMF method is also likewise known for the controlled remelting of or metal forming on single-crystal components (EP 1476272 B1).
However, the production of abrasive protective coatings with an epitaxially solidified intermediate coating on a single-crystal basic material is not yet known.